There appears to be a regular night train going from Sofija
to Thessaloniki
that leaves at 11:00 PM and arrives in Greece at 7:13 AM. I
found this by
using the Swiss timetable Website at
I always have been able to find the schedule I am looking for
there.

As for your trip to Hallstatt. The best route and travel time
(5 hours)
appears to be the 7:30 AM train out of Innsbruck with one stop
in Wels. It
takes a little effort to get to Hallstatt, so I would suggest
getting an
early start. Don't forget that the train station is on the
other side of
the lake from the town and the ferry across does not wait long
at the
dock. Have a good trip.